Elastomeric polymers are utilized in a wide variety of applications for which non-elastic polymers are totally unsuited. For example, in articles of clothing, elastomeric polymers are utilized as neck, wrist, waist, ankle and head bands or in elastic laminate structures used where a certain degree of freedom of movement is required. As the garment is worn, the elastic polymer band or elastic laminate structure must have enough "unload power" to hold the garment in place. When the garment is not worn, it is desirable that the elastic polymer band or a plastic laminate patch have a low "residual set" so that the elastic polymer band or elastic laminate structure of the garment returns essentially to its original shape as the garment is worn over a long period of time. In terms of garment applications, these constraints dictate which of those elastomeric polymer compositions now known can be fabricated into article forms which can be placed into applications in garments.
"Unload power" is an important elastic tensile property in elastic applications, particularly for use in garments. For example, in diaper applications, the unload power of an elastomeric article provides an indication of the retractive force which holds the elastomeric article which is part of the diaper leg of the garment against an infant's body. In all elastomeric materials, the unload power is lower than the "load power" or modulus (the force required to extend the strip). This difference shows up as a "hysteresis" (i.e. the force to extend is different than the force to hold in place) and is larger for synthetic elastomers than for natural rubber. "Residual set" refers to the change between the length of an elastomeric material before and after its extension to a certain length for a certain time for a certain number of cycles. For example, the residual set is the percent change in length of an elastic film after extension of the film to 150-200 percent of its initial length through 2-6 cycles. Each cycle would consist of extending the film to 150-200 percent of its initial length, holding the film extended for a time period, releasing the extending force, and allowing the film to return to its original shape for a time period.
Typical elastic materials utilized for clothing applications include polyurethanes, ethylene-propylene rubbers (EP or EPR), including ethylene-propylene-diene terpolymers (EPDM), and natural rubbers.
Polyurethanes have the desired unload power, residual set and repeatability for use in most garment applications. However, Polyurethanes have a relatively high specific gravity, which results in a lower yield of polyurethane articles hence a higher cost as compared to comparable lower specific gravity polymers. Furthermore, where the garment is to be used once and discarded, such as with surgical garments or disposable diapers, polyurethane's as an element thereof are over-engineered for the desired use of that garment and thus overly expensive. For garment applications, conventional EP's and EPDM's have very poor intrinsic physical properties for such applications and generally must be blended with a plastic material such as low density polyethylene, linear low density polyethylene or ethylene vinyl acetate copolymers. Ideally, in such garment applications an EP and/or EPDM which does not require blending is desired.
Elastomeric SBC's (styrenic block copolymers) also have the disadvantage in that they cannot be drawn to the desired thickness and also have to be blended with other materials, polymers, such as EVA's, EMA's, or LDPE plastomers to achieve the desired processability.
For polyurethanes, EP's and EPDM's, to get an "accordion" shape or "gather" formation, such as around the leg opening of a disposable diaper, it has been necessary to expose that portion of the overall garment article to heat to cause shrinkage of the elastomeric article of the garment. Conventional, elastic materials require a relatively high shrinkage temperature. In addition, many of them require prestretching for good gather formation. Exposing a garment article to such a relatively high temperature may be detrimental to the overall properties of the garment. Further, the commonly used elastic materials generally have to be melt glued to achieve bonding to the garment, such as to a polyolefin layer in the garment. Controlled heat bonding to such a polyolefin substrate, if possible, would be advantageous.
A need exists for an elastic article that can be economically utilized in disposable garment applications. There also exists a need for use in garment applications for an elastic article with a relatively low shrinkage temperature. Additionally, there exists a need for an elastic article that can be heat bonded/sealed to polyolefins. A need still exists for elastic articles of optimum quality for particular uses.
The invention generally relates to elastic materials made from metallocene catalysts. Such materials are produced by conventional blown or cast film processes, as well as cast embossed. Thus, eliminating costly post extrusion or compounding steps.